1. Field of the Invention
This invention relates to a three-dimensional optical memory medium in which information is recorded by three-dimensionally and locally forming a region, in which valence of luminescent ions, such as rare earth element ions, has been varied, in a luminescent-ion-containing solid medium, and thereby varying emission wavelength and luminous intensity of the luminescent ions, relative to surrounding luminescent ions. The invention further relates to a process for producing the three-dimensional optical memory medium by recording information in the solid medium.
2. Description of the Related Art
In recent years, the necessity of increasing the capacity of an optical memory element has risen, and the research into and the development of the techniques concerning the matter are being energetically forwarded. Regarding such techniques, a method of increasing the capacity of an optical memory element by shortening a wavelength of a writing laser beam, and thereby increasing a recording density has been studied. However, since an optical material increases its absorption rate with a shortening of wavelength of such a laser beam, around a half of a currently available wavelength of 780 nm will be a limit level for the shortening of the wavelength. Since a bit size is limited due to a diffraction limit at the wavelength, it is considered that an upper limit for increasing the recording density will be around four times the level currently attainable.
Under the circumstances, the increasing of the capacity of an optical memory element by increasing a spatial dimension of recording from two dimensions to three dimensions, instead of increasing a relative recording density by reducing the size of a recorded region, is being examined. The methods belonging to this system include a method of recording information three-dimensionally by using a photochromic material the transmittance of which varies by the irradiation of light, and a method of three-dimensionally generating variation of refractive index by using photorefractive crystals. However, in the method using a photochromic material, an organic material is liable to cause the photochromic material to encounter the occurrence of deterioration and decomposition thereof ascribed to heat and light. Furthermore, the recording condition varies with the lapse of time, and even the reading light causes an optical reaction due to a high sensitivity of the photochromic material to progress, and the recording condition to vary, these being the drawbacks of this method. On the other hand, in the method using photorefractive crystals, the recording condition becomes different in different axial directions of the crystals during a recording operation since the photorefractive crystals have optical anisotropy.
The study of increasing the capacity of an optical memory element by multiplexing a wavelength of the light used for information reading and writing operations and thereby increasing the recording density per spot is also being forwarded. The techniques belonging to this system include photochemical hole burning (PHB). The photochemical hole burning techniques utilize an enlarged width of an optical absorption spectrum of active centers in a system, in which an organic pigment or rare earth element ions are dispersed as active centers in a transparent solid medium made of glass, a polymer, ionic crystals and metal oxide crystals, in comparison with a width (uniform width) which the spectrum originally has due to the ununiformity of the medium. Namely, when a laser beam of a small width is applied to a specific wavelength within an ununiform width of the spectrum, the absorption of the irradiated wavelength is saturated to put the absorption spectrum in a holed state. According to this method, it is said that a multiplicity of not smaller than 103 per spot can be attained in principle, and that a recording density can be increased to 1011 bits/cm2.
However, most of the PHB phenomena are observed only at an extremely low temperature of not higher than xe2x88x92200xc2x0 C., and such a phenomena does not occur at room temperature, and this poses problems. In recent years, a PHB phenomenon has been observed even at room temperature (refer to K. Hirao et al, J. Lumi., 55,217 (1993)) but problems including a low multiplicity and a low production efficiency are left unsolved. A three-dimensional optical memory glass which solves such problems is taught in U.S. Pat. No. 5,694,249 corresponding to Japanese Patent Laid-Open Publication No. 8-220688. This three-dimensional optical memory glass is stable against heat and light, and does not have optical anisotropy. When a pulsed laser beam is condensed or convergently applied to the interior of a glass matrix as the glass matrix is three-dimensionally scanned therewith, photo-induced variation of refractive index occurs in fine spots, and information is recorded as spatial refractive index distribution. This method enables recorded information stable for a long time against heat and light and superior in weather resistance, and enables the recording capacity of an optical disk to be increased.
Optical memory glass utilizing variation of an absorption coefficient of a spot region by forming a single or plural spots in which the particle dispersion condition is locally varied by convergently applying a pulsed laser beam to the interior of a particle-dispersed medium is reported in Japanese Patent Laid-Open Publication No. 11-232706.
The three-dimensional optical memories disclosed in Japanese Patent Laid-Open Publication Nos. 8-220688 and 11-232706 are characterized in that a signal is detected as a reflectance or a transmittance.
However, in the case of the optical memory glass taught in Japanese Patent Laid-Open Publication No. 8-220688, the techniques used therein are confined to inducing refractive index variation by convergently applying a pulsed laser beam to a glass matrix, and the material itself irradiated with the pulsed laser beam is the same. Therefore, a large variation of composition does not occur between the portion of the material in which variation of refractive index occurs and the portion thereof in which variation of refractive index does not occur, and the amount of an induced refractive index variation cannot be increased greatly. In this optical memory glass, variation of transmittance or reflectance caused by variation of refractive index alone is utilized. Due to a small variation of refractive index, a contrast (S/N) in the reading of recorded information may not be increased to a high level.
In the case of the optical memory glass taught in Japanese Patent Laid-Open Publication No. 11-232706, a decrease in an absorption coefficient due to the variation of the number, size and mode of fine particles is utilized, and a reflectance or transmittance is read as a signal. Therefore, it is necessary that the light in an absorption wavelength region of a particle-dispersed medium be used for a reading operation. This may cause a reading contrast (S/N) to be deteriorated as the number of the recording layers increases, and the signal separation between the recording layers to be also deteriorated since an absorption coefficient of a surrounding region is higher than that of the recorded region.
It is therefore an object of the present invention to provide a three-dimensional optical memory medium that is improved in three-dimensional reading accuracy.
It is another object of the present invention to provide a process for producing such medium.
According to the present invention, there is provided a three-dimensional optical memory medium comprising:
a solid medium forming a base substrate of said optical memory, said solid medium comprising luminescent ions having a first valence; and
a plurality of spots which are three-dimensionally distributed in said solid medium, said luminescent ions being contained in said spots and having a second valence different from said first valence as a result of condensing a pulsed laser beam in said solid medium.
According to the present invention, there is provided a process for producing a three-dimensional optical memory medium, said process comprising:
(a) providing a solid medium comprising luminescent ions having a first valence;
(b) condensing a pulsed layer beam to a focal point in said solid medium such that a spot corresponding to said focal point is formed in said solid medium, said spot comprising said luminescent ions having a second valence different from said first valence; and
(c) three-dimensionally scanning said solid medium with said pulsed laser beam such that a plural number of said spot are formed three-dimensionally in said solid medium, thereby producing said three-dimensional optical memory medium.